The present invention relates to the field of display technologies. In particular the present invention discloses a signal amplifier that may be used in driving Liquid Crystal Display (LCD) panels.
Liquid crystal display (LCD) panels generally operate by having a matrix of electrically controllable liquid crystals that are used to modulate reflected or emitted light. By turning on and off the liquid crystals of the matrix in a particular pattern, an image is rendered across liquid crystal matrix. Liquid crystal display panels are used in many different types of portable electronic devices such as digital cameras, notebook computers, and flat panel television systems.
The liquid crystals in a liquid crystal display panel will lose the capability of modulating light if the liquid crystals are not maintained properly. Specifically, the polarity of the voltage applied to a liquid crystal must be changed within specified time or else the desired chemical characteristics of the liquid crystal will change. Thus, in most liquid crystal systems, the polarity of the analog data is inverted between each successive frame. This liquid crystal maintenance technique is commonly referred to as xe2x80x9cpolarity-inversion.xe2x80x9d
A liquid crystal display panel generally operates with a 5.0 Volt differential. However, liquid crystals need twice that range in order to perform a required xe2x80x98polarity inversionxe2x80x99 that maintains the liquid crystals in the display panel. Thus, the amplifier for driving an LCD panel needs to drive a full 10.0 Volt range. Furthermore, an amplifier for a LCD panel must have a high slew rate in order to rapidly refresh the liquid crystal display panel. Thus, it would be desirable to have a wide voltage bandwidth and high slew-rate amplifier circuit for driving liquid crystal display panels.
The present invention introduces a versatile amplifier circuit of the can he used as a high voltage column driver circuit for a TFT LCD panel. The Amplifier circuit of the present comprises consists of a complementary input stage, biasing switches, and a rail-to-rail output stage. The complementary input stage can operate with a rail-to-rail input. A signal-transfer switch determines which of two differential amplifiers in the input stage will drive the output stage of the amplifier. A biasing signal precharges a capacitor between the gates of output stage. The rail-to-rail output stage utilizes the precharged capacitor to maintain a voltage required to operate the output stage properly.
A polarity signal is used to control the signal-transfer switch. The polarity signal specifies if the lower half of the input stage or the upper half of the input stage is used to drive the output stage of the amplifier circuit. A non-active transistor is kept turned-on above the threshold voltage for quick switching of the output driver. In one embodiment, a coupling capacitor is used between the PMOS transistor and NMOS transistor of the output stage for this purpose. This capacitor maintains the voltage necessary to keep the non-active transistor turned-on and flowing with a minimum current, and prevents the non-active transistor from being turned off. This simple scheme allows the amplifier circuit of the present invention to operate rapidly and in rail-to-rail range.
Other objects, features, and advantages of present invention will be apparent from the company drawings and from the following detailed description.